Interpretive Summary: Palmer amaranth, a pigweed species that is not native to the Southeast US, is one of the most common and problematic weeds Southern agronomic crop production. Since the mid 1990's glyphosate has been extensively used to effectively and economically manage weeds. Cotton growers transitioned away from soil-applied residual herbicides and cultivation for weed control to systems of glyphosate applied multiple times. The selection pressure arising from this unprecedented use of glyphosate over space and time led to the evolution of GR biotypes in Palmer amaranth. Beginning in 2005, numerous Georgia growers were forced to abandon their cotton crops because of their inability to manage GR Palmer amaranth using glyphosate, which had previously provided excellent weed control. These growers now spend up to $130 ha-1 on herbicides, and herbicides alone often do not adequately control GR Palmer amaranth forcing growers to implement hand weeding and/or cultivation. As of 2008, GR Palmer amaranth populations have been confirmed in five states (AR, GA, NC, SC, and TN); the total estimated infested area exceeds 600,000 ha. Future management strategies will rely heavily on an improved understanding of Palmer amaranth biology and ecology, particularly plant population demographics. The development of integrated management approaches using cultural, mechanical, and chemical controls may be the only economically effective option for controlling GR Palmer amaranth.

Technical Abstract:
Since glyphosate-resistant (GR) crops were commercialized, glyphosate has been extensively used to effectively and economically manage weeds. The adoption of GR technology also provided growers with the capabilities needed to rapidly adopt conservation tillage production systems. Selection pressure arising from this unprecedented use of glyphosate over space and time led to the evolution of GR biotypes in Palmer amaranth. Beginning in 2005, numerous Georgia growers abandoned their cotton crops because of their inability to manage GR Palmer amaranth. Growers spend up to $130 ha-1 on herbicides, and herbicides alone often do not adequately control GR Palmer amaranth forcing growers to implement hand weeding and/or cultivation. As of 2008, GR Palmer amaranth populations have been confirmed in five states (AR, GA, NC, SC, and TN), infesting in excess of 600,000 ha. The development of integrated management approaches using cultural, mechanical, and chemical controls may be the only economically effective option for controlling GR Palmer amaranth. The most effective and economical programs for managing GR Palmer amaranth are still those that are preventative in nature. Although resistant Palmer amaranth continues to spread at an alarming rate, most growers have become more aggressive in their management programs. It is hopeful that these aggressive programs will reduce the rate of spread and begin to slowly reduce the number of resistant seeds currently present in soil seedbanks of heavily infested fields.